On 06/17/2014 07:03 AM, David Laight wrote:
From: Peter Hurley
I don't understand the second half of the changelog, it doesn't seem
to fit here: there deadlock that we are trying to avoid here happens
when the *same* tty needs the lock to complete the function that
sends the pending data. I don't think we do still do that any more,
but it doesn't seem related to the tty lock being system-wide or not.
The tty lock is not used in the i/o path; it's purpose is to
mutually exclude state changes in open(), close() and hangup().
The commit that added this  comments that _other_ ttys may wait
for this tty to complete, and comments in the code note that this
function should be removed when the system-wide tty mutex was removed
(which happened with the commit noted in the changelog).
What happens if another process tries to do a non-blocking open
while you are sleeping in close waiting for output to drain?
Hopefully this returns before that data has drained.
tty_open() should be trylocking both mutexes anyway in O_NONBLOCK.